The present invention relates to the bromination of cyclic compounds.
Various observations have been made by the prior art regarding reactions between bromine and 3,3',5,5'-tetra-substituted diphenoquinone carried out in both the presence and the absence of inert liquid solvents. For example, U.S. Pat. No. 3,929,908 discloses the bromination of 3,3',5,5'-tetra-substituted diphenoquinones in the absence of any inert diluent at about ambient room temperature (25.degree. C.) while cooling the reaction media and controlling the rate of addition of the diphenoquinone so as to avoid an uncontrolled exothermic reaction. Said process requires large excesses of bromine in order to maintain a liquid reaction mixture, and produces 3,3',5,5'-tetrabromo-2,2',6,6'-tetra-substituted-4,4'-biphenol (TTB). The use of large excesses of bromine is disadvantageous in many ways. For example, the use of large excesses of bromine can give products which contain undesirably large amounts of occluded bromine. Example 7 of the patent discloses the preparation of 3,3',5-tribromo-2,2',6,6'-tetramethyl-p,p'-biphenol (Tri-TB) with a selectivity of 76 percent as determined by gas chromatograph; the calculated yield is approximately 62 percent.
U.S. Pat. No. 4,058,570 discloses a process for the preparation of TTB wherein bromine is added to a 3,3',5,5'-tetra-substituted diphenoquinone none (TSDQ) at a temperature of less than 20.degree. C. in the presence of a liquid diluent, then evolving HBr at a temperature of at least about 15.degree. C., heating the resulting reaction mixture at elevated temperatures to complete the bromination reaction, and recovering TTB in a yield of at least 70 percent. The patent teaches that molar Br.sub.2 :TSDQ ratios of 6.5:1 are operable, but prefers ratios of 15:1 or higher. The patent does not disclose the production of Tri-TB.
Polybrominated tetra-substituted biphenols have several uses, many examples of which are listed in the patents cited hereinabove. For example, said biphenols can be employed as flame-retardant additives for resinous materials, and can be copolymerized to give flame-retardant polymers which can be readily formed into films, sheets, fibers, laminates, etc. However, TTB is poorly soluble in many organic solvents, and therefore is difficult to handle in certain applications. As stated in JFF/Fire Retardant Chemistry, V. 2, pp. 183-193 (1975), the solubility of TTB in acetone at 25.degree. C. is only 3.2 g per 100 cc of solvent.
In view of the deficiencies of TTB and the prior art processes for its preparation, it would be desirable to have a process which could produce in high yield a polybrominated tetra-substituted biphenol having increased solubility in organic solvents, and which could be operated without requiring large excesses of bromine.